Recent years have seen the development of diagnostic techniques that allow the practicing clinician to obtain high fidelity views of the anatomical structure of the human body. Imaging systems such as computed tomographic (CT) x-ray imagers, positron emission tomographic (PET) scanners, single photon emission computed tomography (SPECT) scanners and nuclear magnetic resonance imaging (MRI) machines have provided clinicians with the ability to improve visualization of the anatomical structure of the human body without surgery or other invasive techniques. In lieu of exploratory surgery, the patient can be scanned by these imaging systems, and the patient's anatomical structure can be reproduced in a form for evaluation by a trained doctor. A problem associated with such scanning techniques concerns the accurate selection and comparison of views of identical areas in images that have been obtained by imagers at different times or by images obtained essentially at the same time using different image modalities, e.g., CT, MRI, SPECT, and PET. This problem has two aspects. First, in order to relate the information in an image of the anatomy to the anatomy itself, it is necessary to establish a one-to-one mapping between points in the image and points of anatomy. This is referred to as registering image space to physical space.
The second aspect concerns the registration of one image space onto another image space. The goal of registering two arbitrarily oriented three dimensional images is to align the coordinate systems of the two images such that any given point in the scanned anatomy is assigned identical addresses in both images. The calculation of the rigid body transformation necessary to register the two coordinate systems requires knowledge of the coordinate vectors of at least three points in the two systems. Such points are called "fiducial points" or "fiducials," and the fiducials used are the geometric centers of markers, which are called "fiducial markers". These fiducials are used to correlate image space to physical space and to correlate one image space to another image space. The fiducial markers provide a constant frame of reference visible in a given imaging mode to make registration possible.
The general technique for using fiducial markers to obtain registration of image data across time is set forth in U.S. Pat. Nos. 4,991,579 and 5,142,930, the contents of both of which are incorporated herein by reference. Briefly, these patents teach implanting within a patient a series of three fiducial markers whose location can be determined in the image space of an imager.
Broadly speaking, image markers can be either temporary or permanent with respect to the duration of their placement within the human body. Permanent markers are placed entirely beneath the epidermis of the skin for extended periods of time. Temporary markers (more fully described in the parent application Ser. No. 08/017,167) have two parts: a base that is implanted into bone, and a temporary image marker portion that is attached to the base for brief intervals of time. In the temporary marker, the image marker portion protrudes from the skin.
In both the temporary and the permanent markers, the marker portion may take the form of a hollow container that is charged with aqueous imaging agents to provide imaging capability in the desired imaging modality or modalities. Parent application Ser. No. 08/017,167 (the contents of which are incorporated herein by reference) more fully discusses the structure of each type of marker and the imaging agents which can be used with therewith.
Whichever type of marker is employed, its precise location, or more accurately, the precise location of the geometric center of the imageable portion of the marker must be determined with respect to a defined external coordinate system in physical space. With respect to permanently implanted markers, ultrasound can be used to determine non-invasively the location of the fully implanted marker. Other techniques can be employed with respect to temporary, externally protruding markers. One method involves bringing the tip of an external probe whose location in physical space is known into proximity with the marker itself. However, this may result in significant errors in the location of the precise volumetric centroid of the imaging portion of the marker. There remains a need for a technique for locating the center of a temporary fiducial marker that is simple to practice and which is very accurate.